This invention relates to FM receivers, and particularly to FM receivers adapted to receive weak signals in the presence of noise such as encountered in reception of television signals transmitted via satellite. Specifically, the present invention relates to receivers for satellite transmitted television signals, i.e., television receive only (TVRO) receiver utilizing noise threshold extension circuitry of the oscillating limiter type.
Information signals such as television signals having a variety of frequency components may be frequency or angle modulated onto a carrier signal of predetermined frequency. A typical television signal includes among other information, a color burst or chroma signal at a frequency of about 3.58 MHz, and may further include audio subcarriers in a range of frequencies between approximately 5.4 MHz and approximately 8.5 MHz. The resulting frequency modulated (FM) signal is of a predetermined bandwidth centered about the frequency of the carrier signal. The FM signal may be transmitted from an earth-bound transmitter to an orbiting satellite and subsequently retransmitted from the satellite to earth receiving stations. The earth receiving station might normally include a reflector antenna configured to receive the satellite signal. The reflector antenna is typically coupled to a low noise amplifier which is further coupled to a receiver such as a FM TVRO receiver. The FM TVRO receiver is designed to demodulate the television signal from the carrier signal. To that end, the FM receiver will typically include circuitry to heterodyne, super-heterodyne, or otherwise mix the signal received at the antenna dish with one or more local oscillator signals to produce an intermediate frequency (IF) FM signal which can more easily be operated upon by conventional receiver circuitry. The IF signal is still an FM signal, albeit at a lower frequency as is well understood. That is, the FM signal transmitted by the satellite may have a certain carrier frequency in the several gigahertz range whereas, in television satellite communication systems, the IF signal may typically be centered about 70 MHz with a bandwidth of approximately 30 MHz, i.e., about 55 MHz to 85 MHz.
The IF signal is subsequently coupled to a demodulator or detector where it is demodulated to reproduce as nearly as possible the original modulating or baseband signal. This resultant demodulated, or baseband, signal is provided to a television monitor for viewing, or may be further process as desired.
In a television satellite transmission system, the baseband signal is, ideally, 0 to 8.5 MHz and includes the video, audio subcarrier and related information signals only. Within the baseband, the continuous video region from below about 30 Hz to about 4.2 MHz is of primary interest in the demodulation of 525 line television formats, such as NTSC, whereas for 625 line television format, such as PAL, the continuous video region of primary interest is from below about 25 Hz to about 5.0 MHz. The region lying between the upper end of the video baseband (about 4.2 MHz or about 5.0 MHz) to the typical baseband upper limit of about 8.5 MHz is normally used for the transmission of relatively narrow band FM subcarriers, a common format having the same transmission parameters as broadcast FM transmissions. These subcarriers are normally detected by suitable narrow band detectors, and because of the reduced bandwidth typically associated therewith, are more resistant to the effects of noise. Wideband subcarriers may also be encountered.
In reality, due to the nature of the satellite communication system involved, the FM signal appearing at the input to the receiver of the earth receiving station is typically extremely weak and accompanied by a substantial amount of electrical noise. This condition is caused, in part, by the fact the signal transmitted by the satellite transmitter must travel a great distance to reach the earth-bound receiver. As a result, the strength of the information portion of the signal received may be so weak as to not be intelligible after demodulation. Compounding the difficulty of receiving such weak signals is the unavoidable addition of terrestrial noise to the signal due to objects with non-zero temperature in the view of the reflector antenna. Objects with non-zero noise temperature are sources of electrical noise which can be received by the reflector antenna. In addition to the terrestrial noise, a variety of other unavoidable electrical noise sources are commonly encountered in typical communication systems, as well as in satellite communication systems. The net effect is that the receiver must extract an extremely weak information signal in the presence of strong noise if satisfactory results are to be achieved. The foregoing, and the essentially triangular spectral distribution of noise in the baseband, result in a baseband or demodulated signal which is not a true reproduction of the original television signal but will likely also contain a great deal of noise, i.e., spurious signals. Such noise can degrade picture quality and/or audio fidelity and may even preclude detection of the information content of the signal.
As mentioned, an FM signal has a center frequency which is the carrier frequency. Ideally, the frequency of the FM signal will thus vary about the center or carrier frequency but the amplitude of the signal will not vary. Hence, it can be assumed that amplitude variations (AM) on the received signal are noise. To eliminate such AM noise, it is common practice to employ an amplitude limiter between the IF stage of the receiver and the subsequent demodulator stage. The amplitude limiter operates to limit amplitude variations appearing on the FM signal as appears from the IF stage thus reducing the AM noise therein, and preventing its conversion to the baseband output by detector imperfections.
Where the strength of the information signal received is large compared to the noise in the signal, an amplitude limiter alone will usually suffice to sufficiently suppress the AM noise. However, where the information signal strength is weak compared to the noise, reduction of the AM noise by the amplitude limiter will be insufficient for quality picture reception and/or may adversely affect the weak information signal precluding proper demodulation.
As a measure of information or the ratio of modulation information to noise, it is typical to determine the carrier-to-noise ratio, or CNR. In terms of CNR, about at 12 to 14 dB and higher, the amplitude limiter is alone sufficient to suppress AM noise. On the other hand, at CNR levels below about 12 dB the limiter's capability is usually not adequate to properly suppress the noise without also affecting the information signal.
As recognized in U.S. Pat. No. 3,909,725 to Baghdady, at such low CNR, the amplitude limiter's performance can be greatly improved by providing regenerative (i.e., in-phase) feedback around the amplitude limiter. Regenerative feedback results in improved reception by suppressing the noise without degradation of the information signal. Thus, in U.S. Pat. No. 3,909,725, which is incorporated herein by reference, there is disclosed a feedback amplifier and filter configured to provide in-phase feedback around the limiter in the frequency band of interest. The filter is typically a bandpass filter with a bandpass wide enough to pass the entire FM signal containing the modulating signal. Such regenerative feedback permits better reception of weaker information signals in the presence of noise, and, subsequently, more satisfactory demodulation for viewing purposes, than previously possible. Hence, the lower limit or threshold of CNR at which proper reception can occur is extended. This phenomenon or technique is sometimes, therefore, referred to as threshold extension.
When regenerative feedback around the limiter is employed, the circuit will normally tend to oscillate in the absence of an input signal. Hence, a limiter having regenerative feedback is often referred to as an oscillating limiter. This self-induced oscillation has the added benefit of providing a squelch to the receiver as described in the aforesaid Baghdady patent.
The Baghdady oscillating limiter concept appears to substantially lower the CNR threshold at which proper demodulation can occur, however, its boundary conditions for proper operation are exceeded under many satellite television modulation conditions. Subsequent developments with oscillating limiters have attempted to further extend the boundary conditions by providing for electrical tuning of the feedback filter, referred to as an electrically tunable bandpass filter. Thus, in U.S. Pat. Nos. 4,035,730 and 4,101,837, the feedback filter is an electrically tunable bandpass filter with a bandwidth apparently narrower than the IF bandwidth and having a center frequency nominally set at the IF center frequency (e.g., 70 MHz). These two patents have apparently proceeded on the assumption that the boundary would be further extended by tuning or "steering" the center frequency of the filter so that it tracks or matches the frequency of the FM input while trying to cause the tuning to ignore the noise in the signal. The patents describe steering as follows: the FM signal is demodulated to provide a baseband signal (and noise); the baseband signal is then filtered such that the high frequency or noise components are cut off and the filtered signal coupled to the feedback filter in an effort to cause the center frequency of the feedback filter to substantially match the frequency of the received IF FM signal; specifically, the steering is to apparently be done with at least the chroma (3.58 MHz) portion of the baseband signal but supplied to the feedback filter with zero phase relative to the limiter output. The latter patent, U.S. Pat. No. 4,101,837, emphasizes this point by describing the loop delay (between limiter output and filter control input for the steering signal) as desirably being 360.degree., i.e., substantially in-phase. If such attempts have worked at all, they have met with only marginal success in raising the boundary modulation conditions at which the oscillating limiter will improve reception. Moreover, an oscillating limiter which uses a feedback filter having a bandwidth less than the bandwidth of the FM signal wherein the center frequency is steered in response to the modulation of the FM signal apparently also interferes with proper demodulation at high CNR. Hence, in one prior art unit, the feedback filter is electronically decoupled from the limiter at CNR greater than about 12 dB CNR. Accordingly, such attempts to steer the filter may even result in poorer quality reception rather than improved reception.
Additionally, with the advent of satellite communications, a further problem has been encountered. Ideally, each satellite transponder which is set to a particular channel will operate at the same nominal or carrier frequency. That ideal is not always achieved. Hence, the signal to be received from one satellite may be at the correct nominal frequency whereas the signal to be received from a second satellite may be offset slightly in frequency due to drift or the like. Additionally, the receiving system may operate with some unwanted frequency offset of its own due to changes caused by temperature fluctuation such as in equipment mounted at the reflector antenna.